Crystalline silicon (including multi- and mono-crystalline silicon) is the most dominant absorber material for commercial photovoltaic applications. The relatively high efficiencies associated with mass-produced crystalline silicon solar cells, in conjunction with the abundance of material, garner appeal for continued use and advancement. But the relatively high cost of crystalline silicon material itself limits the widespread use of these solar modules. At present, the cost of “wafering,” or crystallizing silicon and cutting a wafer, accounts for about 40% to 60% of the finished solar module manufacturing cost. If a more direct way of making wafers were possible, great headway could be made in lowering the cost of solar cells.
There are different known methods of growing monocrystalline silicon and releasing or transferring the grown wafer. Regardless of the methods, a low cost epitaxial silicon deposition process and a high-volume, production-worthy low cost method of release layer formation may be prerequisites for wider use of silicon solar cells.
Porous semiconductor (PS) formation is a fairly new field with an expanding application landscape. The viability of this technology in solar applications may hinge on the ability to industrialize the process to large scale (at low cost), requiring development of very low cost-of-ownership, high-productivity porous silicon manufacturing equipment.
PS has been used in MEMS (micro-electro-mechanical systems) and related applications, where there is a higher tolerance for cost per unit area of the wafer than solar PV. The microelectronics industry achieves economy of scale through obtaining greater yield by increasing the number of die (or chips) per wafer, scaling the wafer size, and enhancing the chip functionality (or integration density) with each successive new product generation. In the solar industry, economy is achieved through the industrialization of solar cell and module manufacturing processes with low-cost, high-productivity equipment. Further economies are achieved through price reduction in raw materials through reduction of materials used per watt output of solar cells.
Some typical precursor chemistries for PS are: HF (49% in H2O typically), IPA (and/or acetic acid) and DI H2O. IPA (and/or acetic acid) serves as a surfactant and assists in the uniform creation of PS. Additional additives may be used to enhance the electrical conductivity of the electrolyte, thus reducing its heating through ohmic losses. Mixtures of HF and chemicals other than IPA can be readily employed by those skilled in the art.
In order to achieve the necessary economy for solar, process cost modeling is studied to identify and optimize equipment performance. Three categories of cost make up the total cost picture: fixed cost (FC), recurring cost (RC) and yield cost (YC). FC is made up of items such as equipment purchase price, installation cost and robotics or automation cost. RC is largely made up of electricity, gases, chemicals, operator salaries and maintenance technician support. YC may be interpreted as the total value of parts lost during production.
To achieve the cost-of-ownership (CoO) numbers required by the solar field, all aspects of the cost picture must be optimized. The qualities of a low cost process are (in order of priority): 1) high productivity, 2) high yield, 3) low RC, and 4) low FC.
Designing highly productive equipment requires a good understanding of the process requirements and reflecting those requirements in the equipment architecture. High yield requires a robust process and reliable equipment, and as equipment productivity increases, so too does yield cost. Low RC is also a prerequisite for overall low CoO. RC can impact plant site selection based on, for example, cost of local power or availability of bulk chemicals. FC, although important, is diluted by equipment productivity.
With the above said, in summary, high productivity, reliable, efficient manufacturing equipment may be a prerequisite for low cost solar cells.
Achieving low RC requires efficient use of chemicals. In wet processes, “drag out” or chemical carried out of the reaction chamber, must be rinsed off the wafer. With a greater amount of “drag out,” a correspondingly greater amount of rinse water is required to clean the wafer. Both of these factors add to CoO. Moreover, one must minimize the aging of chemicals so that they can be reused and/or recycled over an extended period.